[clang] Poor error recovery when performing pack expansion on expressions

[mizvekov]

Repro: https://godbolt.org/z/K6s7W6d8z

template <class> struct A;
template <class> using B = char;
template <class Cs> int C{ A<B<Cs>>{}... };

We incorrectly diagnose a pack expansion without unexpanded parameter packs:

<source>:3:28: error: implicit instantiation of undefined template 'A<char>'
template <class Cs> int C{ A<B<Cs>>{}... };
                           ^
<source>:1:25: note: template is declared here
template <class> struct A;
                        ^
<source>:3:38: error: pack expansion does not contain any unexpanded parameter packs
template <class Cs> int C{ A<B<Cs>>{}... };

[llvmbot]

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